HTC Thunderbolt User Manual page 178

178 Email
Viewing the unified inbox of all your email accounts
You can view email from all your accounts just in one place without
having to switch to different accounts.
Just tap the bar that shows the current email account, and then tap
All accounts.
Email messages are color coded so you can easily distinguish which
email accounts they belong to.
Up to 15 email accounts can be shown in the unified inbox.